Living, non-dividing cells (such as oocytes) can still gather damage to their DNA through a variety of ways (mutagenic substances, radiation, etc.). Misrepair of DNA damage by the cells' own repair mechanisms can form mutations which can be inherited, if the mutations don't make the oocyte non-viable of course.

Agreed with the above! NHEJ is often used for the insertion of 'random' nucleotides for generating gene knockouts. HR can more efficiently be used for targeted insertions when also introducing a repair template along with the CRISPR complex that has homology arms that match the target splice site.